The wheel electronic units (called “wheel units”) currently have the functionality of informing a centralized electronic control unit (hereinafter called “central unit”) concerning various parameters relating to the wheel, such as the tire pressure, the temperature of the gas inside of the tire or wheel temperature, the footprint of the tire, for example the length of this print along the plane of the wheel. These wheel units 13 (cf. FIG. 1) are generally fitted to each wheel 11 of the vehicle 10 (cf. FIG. 3) and are generally fixed inside the tire against its tread, and are provided with means that make it possible to record the moment when this tread comes into contact with the ground S and the moment when it leaves the ground S, by the induced radial displacement of the tread. These means are, for example, an accelerator or an impact sensor or similar associated with the wheel unit 13, which measures, in the case of the accelerometer for example, the difference in radial acceleration when it enters into contact with the ground S and leaves this contact.
The wheel unit 13 is provided with at least one pressure sensor 17 (often also a temperature sensor), and also comprises (in a nonlimiting manner):                a microprocessor,        an antenna 15 for receiving low-frequency LF signals,        an antenna for emitting radiofrequency RF signals (not represented) (cf. FIG. 3), and        a battery (not represented) powering all of these elements.        
The wheel unit 13 can also comprise other sensors as cited above, for example an accelerometer.
The central unit 12, mounted on the vehicle V, is for its part provided with at least:                a radiofrequency receiving antenna 16 (cf. FIG. 3),        a computer with microprocessor,and it is linked electrically to four low-frequency emission electronic units 20 (cf. FIG. 3), that is to say to four emitting units 20.        
This central unit 12 is powered by the battery of the vehicle V.
Each of the emitting units 20 is situated in a fixed manner on the vehicle V close to a wheel unit 13.
Each emitting unit 20 comprises a low-frequency emission antenna, and a microcontroller linked electrically to said emission antenna. The emitting unit 20 can, in certain cases, comprise only the low-frequency emission antenna, linked electrically to the computer, situated in the central unit 12.
At fixed frequency, the central unit 12 controls the sequential sending of low-frequency LF interrogation signals via the emitting units to each of the wheel units 13.
Each wheel unit 13 then transmits sequentially, one after the other, radiofrequency RF signals (generally high frequency) to the central unit 12 in order for the latter to inform the driver of any abnormal measured parameters, and also in order for this central unit 12 to be able to locate the position of each wheel unit 13 on the vehicle V throughout the life of the tire associated with the wheel 11, and do so even if the position of the wheel 11 is changed on the vehicle V.
During the rotation of a wheel 11, and over one revolution of the wheel 11, there are generally parts of trajectories of the wheel unit 13 in which the reception by the wheel unit 13 of the LF signals sent by the closest emitting unit 20 does not work, called low-frequency rupture zones Zo (cf. FIG. 1), which correspond to an RF signal reception rupture zone. This can be true for each wheel 11 of the same vehicle V in a different and unpredictable manner. There is, however, by convention, a limited acceptance of the number and the duration of the rupture zones Zo for a wheel 11 revolution. For example, a rupture zone Zo of approximately 1° maximum is generally accepted for each wheel 11 revolution. In certain particular cases, this number may be raised to two rupture zones Zo each of the order of 1° maximum for each wheel 11 revolution.
FIG. 2 illustrates an example of intensity RFi of the low-frequency LF signal received by the reception antenna 15 of the wheel unit 13 according to the angle of rotation θ of the wheel, over a complete wheel revolution (360°), also called reception pattern. In this example, the rupture zone Zo appears in the regions of 225°, the intensity RFi of the low-frequency LF signal is then insufficient for the wheel unit 13 to receive the LF signal.
A vehicle V without rupture zone Zo and in which the rate of LF reception by the wheel unit 13 reaches 100% is in fact not representative of the reality. In effect, the wheel unit 13 comprises a single low-frequency reception antenna 15, and the emitting unit 20, for its part, has a single low-frequency LF signal emitting antenna, generally in the form of a longitudinal copper coil surrounding a ferrite, which emits LF signals in a single direction. The rupture zone Zo corresponds to a momentary position of the wheel unit 13 on the wheel 11, for which the orientation of the wheel unit 13 is such that the direction of reception of the receiving antenna 15 of said wheel unit 13 does not allow the reception of the low-frequency LF signals sent by the emitting unit 20.
One prior art solution consists in equipping the emitting unit 20 with a second emission antenna, fixed at right angles to the emitting antenna already present in the emitting unit 20. The two antennas have emission directions at right angles. This makes it possible to increase the probability of reception of the LF signals by the wheel unit 13 and reduce the rupture zone Zo. With a second antenna, the rate of reception of the LF signals by the wheel unit 13 comes close to 100%.
However, the addition of a second low-frequency antenna in the emitting unit 20 with its associated electronic control circuit represents a not-inconsiderable cost. The emitting unit 20 also becomes more bulky because of the additional space allocated for the second antenna, at right angles to the antenna already present, so its incorporation in the vehicle V becomes less easy.
Another prior art solution consists in equipping the wheel unit 13 with a second LF signal receiving antenna, at right angles to the receiving antenna 15 already present in the wheel unit 13. However, this addition represents a real bulk drawback, and the incorporation of the wheel unit 13 in the tire 11 against the tread becomes difficult.